The common emitter forward current gain (beta) of a bipolar transistor is not constant. Beta varies according to environmental factors, such as temperature and radiation level. Beta also varies according to operating conditions, such as collector current level and collector voltage level. Moreover, beta varies with semiconductor material and process variations, so that the beta of transistors on one semiconductor wafer can vary widely from that of the transistors fabricated on another semiconductor wafer.
Current mirror amplifiers (CMA's), which reduce the dependence of amplifier gain on beta, utilizing transistor geometry ratios, are well known. However, in these arrangements, the finite base currents drawn by the component bipolar transistors tend to introduce a base-current error term into a CMA transfer function such that the overall CMA gain is still a function of beta. The adverse effect of such base current error on CMA gain is exaggerated when lower beta transistors (e.g. such as lateral PNP transistors) are used, or when higher CMA current transfer ratios (e.g. current gains of ten or more) are desired.
The base-current error components in a CMA can be understood with reference, for example, to a simple CMA comprising first and second transistors, wherein a direct connection from the collector to the base electrode conditions the first transistor to conduct the input current. The base current for both first and second transistors is supplied by the amplifier input current via such collector-to-base connection. However, the amplifier output current at the collector electrode of the second transistor supplies no base current. As the beta of the individual transistors decreases, an increasing portion of the input current is diverted as base current to operate the first and second transistors. Thus, the gain of such simple CMA decreases as beta decreases. Such gain variations are particularly undesirable in drivers required to provide at least a minimum current for apparatus such as may be employed in automobiles since in such applications wide environmental variations are often encountered.